LED light sources have the inherent potential to provide the brightness, output, and operational lifetime of conventional light sources. Unfortunately, an LED light source produces light in a semiconductor material, often referred to as an LED die, and it can be difficult to extract light efficiently from the LED die without substantially reducing brightness or increasing the apparent emitting area of the LED light source. This difficulty is often due to the large refractive index mismatch between the LED die (refractive index, n, 2.4 or greater) and air (n=1) such that much of the light generated in the LED die is totally internally reflected and cannot escape the die thus reducing brightness.
Previous approaches for extracting light from LED dies include the use of encapsulants which encapsulate the LED die. Encapsulants typically have a higher refractive index than air (n˜1.5 for typical epoxy encapsulants) such that the refractive index mismatch at the interface of the die with its surroundings is reduced. As a result, less light is wasted and the light source is brighter. There is, however, need for further improvement in efficiency and brightness. In addition, encapsulants are susceptible to large temperature shifts from heat generated at the LED die which causes stress that can damage the die over many temperature cycles. Encapsulants can also yellow and otherwise degrade over time.
Extractors may also be used to extract light from LED dies. These optical elements are typically transparent polymers or glasses and are disposed in contact with or in close proximity to the surface of the LED die. The extractor and LED die are optically coupled such that light is extracted from the LED die. Extractors typically have an input surface sized and shaped to substantially mate with a major emitting surface of the LED die. The use of extractors, however, has been hampered by the difficulty with bonding the extractor and the LED die. Known bonding methods require elevated temperatures (greater than 350° C.) and pressures, both of which may negatively affect the functionality of the LED light source. Known bonding methods also require involved manufacturing processes, such as chemical vapor deposition, various forms of epitaxy, etc., which limits the usefulness of these methods to select applications.